1. Field of the Invention
The present invention relates to a superconductive magnet device, and more particularly to a superconductive magnet device having an intermetallic compound superconductive coil.
2. Description of the Prior Art
A superconductive magnet device has a coil assembly of a superconductive wire which can be essentially made of a superconductive material. However, a composite superconductive wire which is made of a superconductive material around which highly conductive metal such as copper, aluminum or silver is coated is usually used in preparation for the case of quenching of a superconductive state.
The superconductive material may comprise an alloy such as niobium-titanium alloy or niobium-titanium-zirconium alloy, or it may comprise intermetallic compounds such as Nb.sub.3 Sn, or V.sub.3 Ga. The intermetallic compound can show higher critical temperature and higher critical magnetic field. Accordingly, the use of the intermetallic compound has been noted rather than the alloy.
The composite superconductive wire comprising the intermetallic compound, for example Nb.sub.3 Sn, is manufactured by inserting a niobium wire into a tube of copper-tin alloy, reducing a cross-sectional area of the assembly by drawing or extruding, and heat-treating the assembly to form niobium-tin intermetallic compound Nb.sub.3 Sn at an interface of the niobium wire and the copper-tin alloy tube.
The superconductive wire is wound to form a coil which is assembled in the magnet device. In order to facilitate the winding of the superconductive wire on a coil bobbin or a jig and to increase a space ratio of the superconductive wire, the superconductive wire usually has a rectangular cross-section, and it is wound on the coil bobbin with a longer edge side of the rectangle being in parallel to a center axis of the coil, that is, the longer edge side being in parallel to the direction of magnetic field. This winding method is hereinafter referred to as a flatwise winding method. Alternatively, such a winding method wherein the shorter edge side of the rectangle is in parallel to the center axis of the coil is referred to as an edgewise winding method. The coil bobbin or jig may sometimes be removed when the coil is assembled in the superconductive magnet device.
The superconductive wire is wound about a center axis of the resultant coil assembly in layers in the direction perpendicular to the center axis to form a coil section. A plurality of the thus wound coil sections are stacked in the direction of the central axis to form the coil assembly. The adjacent layers of each coil section are usually insulated from each other by insulators and the respective spaces between adjacent ones of the coil sections usually define flow paths for liquid helium.
The composite superconductive wire having the rectangular cross-section usually exhibits different superconductivity in one direction in the cross-section and a direction perpendicular thereto, although it is greatly influenced by a manufacturing process. Accordingly, the superconductivity differs depending on a particular winding method of the composite superconductive wire. For example, when the superconductive wire is wound flatwise, it frequently occurs that the critical current density in a direction parallel to the magnetic field is smaller than the critical current density in a direction perpendicular to the magnetic field. In this case, the quenching of superconductive state breakdown is apt to occur near the innermost layer of the central coil section where the magnetic field strangth is the highest. It is considered that such a phenomenon is caused by the manufacturing process of the composite superconductive wire having a rectangular cross-section because it is manufactured from a wire having a substantially circular cross-section. In the process of forming the rectangular cross-section wire from the circular cross-section wire, the superconductive material is deformed and finally it loses the circular cross-section. As a result, it is considered that anisotropy occurs between the longer edge direction and the shorter edge direction of the rectangular cross-section for the pinning mechanism and the pinning force of the line of magnetic induction in the superconductive wire. Although the anisotropy of the superconductivity of the composite superconductive wire is not limited to the rectangular cross-section wire, it is apparent that the anisotropy appears in the rectangular cross-section wire, and it is also apparent that the anisotropy appears more clearly in the wire having a higher ratio of the longer edge to the shorter edge of the rectangle, that is, in the wire having higher aspect ratio.
Thus, because of the anisotropy of the superconductivity of the composite superconductive wire having the rectangular cross-section, which anisotropy is imparted by the manufacturing process, the superconductive magnet device having the coil formed by winding such a wire could not be designed to have a sufficiently high current density in view of a risk of quenching of the superconductive state. Particularly in the superconductive magnet device having a compound superconductive coil, a high critical current characteristic of the compound superconductive material could not be effectively utilized because of significant affect by the anisotropy of the superconductivity of the superconductive wire.